Microstructure and thermal cycle resistance of plasma sprayed mullite coatings made from secondary mullitized natural andalusite powder

Natural andalusite powder was calcinated at a high temperature in air to realize secondary mullitization. The resultant secondary mullitized powder was spray-dried and heat-treated to improve sprayable capability. The heat-treated spherical powder was then plasma sprayed onto Ni-based high-temperature alloy (Hastelloy C-276) to form mullite coatings. The chemical composition and phase structure of the as-sprayed and thermally cycled mullite coatings were determined by means of energy dispersive X-ray fluorescence (ED-XRF) and X-ray diffraction. The microstructure of the as-sprayed coatings was analyzed by using a scanning electron microscope; and their porosity, microhardness and bonding strength were measured. Moreover, the phase transition temperature and enthalpy of the coatings were determined by means of differential scanning calorimetry; and their thermal shock resistance was evaluated as well. Results show that the spray-dried and heat-treated powder consists of mullite and a small amount of Al₂O₃; while the as-sprayed mullite coatings are composed of crystalline mullite as the major phase and a small amount of amorphous glass phase. During thermal cycle test, the amorphous glass phase is partially transformed to crystalline mullite, finally leading to failure of the coatings. Whether before or after thermal cycle, the mullite coatings experience phase transition around 980 °C, and the enthalpy of crystallization is determined to be − 141.9 × 10^− 3 J/kg and − 95.48 × 10^− 3 J/kg, respectively. The as-sprayed mullite coatings have a porosity of about 6.0 ± 0.2% and possess good thermal cycle resistance, showing promising prospect in a high-temperature application.